Change test tables in copy2 regression test to be temporary tables.

This prevents autovacuum from reclaiming free space in them and causing
the test's output row order to change, which is causing intermittent
bogus failure reports in the buildfarm.

Backpatch to 8.3.  The issue exists further back, but since autovacuum was
not on by default before 8.3, it's not a problem for buildfarm testing.

src/test/regress/expected/copy2.out

-CREATE TABLE x (
+CREATE TEMP TABLE x (
 	a serial,
 	b int,
 	c text not null default 'stuff',
@@ -172,7 +172,7 @@ I'm null	before trigger fired
 3	after trigger fired
 4	after trigger fired
 5	after trigger fired
-CREATE TABLE y (
+CREATE TEMP TABLE y (
 	col1 text,
 	col2 text
 );

src/test/regress/sql/copy2.sql

-CREATE TABLE x (
+CREATE TEMP TABLE x (
 	a serial,
 	b int,
 	c text not null default 'stuff',
@@ -116,7 +116,7 @@ COPY x TO stdout;
 COPY x (c, e) TO stdout;
 COPY x (b, e) TO stdout WITH NULL 'I''m null';
 
-CREATE TABLE y (
+CREATE TEMP TABLE y (
 	col1 text,
 	col2 text
 );